That lasted for about 15 seconds before I remembered that creating large, non-trivial RIAs generally involves designers, and most designers don't like creating vector art by coding a set of "path" statements, or animations as a collection of key-value pairs and millisecond-based transition times.
That's why tools like Microsoft Expression, and Adobe Illustrator, Catalyst, and Flash exist.
And why Adobe and Microsoft are investing so heavily in the designer-developer workflow: the ability of designers to turn graphics and animations into app skins and interaction which are immediately available to coders.
In order for an open RIA solution to be competitive and realistic -- whether it's open in the pure-browser sense, using JS via dojo.gfx, or Rapael, etc., or whether it's via an open plug-in (Java/JavaFX seems like the closest, though it's not 100% open yet and may never be) -- this full toolchain needs to exist.
We need to be able to export vector art from mainstream design programs such that they can be incorporated as assets into the RIA. It doesn't matter if this is via SVG, XAML, AI/EPS, or something else entirely. What does matter is that the import/export is robust enough that designers -- whose jobs, after all, include making stuff look just right -- are confident that what they design is what end-users will see. The Microsoft and Adobe tools can do this. To date most OSS attempts cannot.
Next up, we need a truly usable, designer-friendly authoring tool for animations and interactions. It is often argued that some standard tools (*cough* Illustrator *cough*) are not paragons of usability themselves. No matter -- it's hard enough to get converts.
Happily, there seems to be emerging some consensus among the big vendors about how these tools should work (both on-screen and in terms of in intermediate data formats). That blueprint lowers the risk and challenge for an open source contender.
The biggest obstacle remaining is a classic open-source triangle-of-trouble:
- The toolchain/workflow will not be viable until it is quite solid, since the commercial alternatives (Flash, mainly) are so entrenched.
- It's hard to get enough contributor man-hours against such a huge project without an active user base.
- Since the user base is not developers, the bootstrapping for #2 that makes many OSS projects work (devs are tolerant -- even excited -- about getting up on an 0.1 release) is unlikely.